Semiconductor laser structure

ABSTRACT

The active layer ( 1 ) and the barrier layers ( 2 ) contain a group III component, a group V component and nitrogen, whereby the active layer is a quaternary material and the barrier layers are ternary materials, or, in order to match the lattice properties of the active layer to the barrier layers, the nitrogen content in the barrier layers is higher. The active layer is preferably InGaAsN, the barrier layers are InGaAsN with higher nitrogen content or GaAsN. Superlattices may exist in the barrier layers, for example, series of thin layers of In x Ga 1−x As y N 1−y  with varying factors x and y, where, in particular, x=0 and y=1.

[0001] The present invention relates to a semiconductor layer structurewhich is suitable for fabricating laser diodes.

[0002] For radiation emission wavelengths of approximately 1.3 μm, useis usually made of the material InGaAsP, preferably on InP substrates.InGaAs as active layer in heterostructures on GaAs is not suitable forthis wavelength range since the band gap (energy band gap) inhomogeneous InGaAs layers would require such a high proportion of indiumthat the layer would become unusable for lasers owing to structuralrelaxation. It has been demonstrated, however, that heterostructures onGaAs can equally be used, in principle, for longer-wave emission if theemitting material is a potential well (quantum well) made of InGaAsN,GaAs layers usually being used as barrier layers above and below theactive layer provided for radiation generation (see e.g. M. Kondow etal.: GaInNAs: A Novel Material for Long-Wavelength Semiconductor Lasers”in IEEE J. Select. Topics Quantum Electron. 3, 719-730 (1997), M. Kondowet al.: “Gas-source MBE of GaInNAs for long-wavelength laser diodes” inJ. Crystal Growth 188, 255-259 (1998) and K. Nakahara et al.: “1.3-μmContinuous-Wave Lasing Operation in GaInNAs Quantum-Well Lasers” in IEEEPhoton. Technol. Lett. 10, 487-488 (1998)). The publication by T.Miyamoto et al.: “A Novel GaInNAs—GaAs Quantum-Well Structure forLong-Wavelength Semiconductor Lasers” in IEEE Photonics TechnologyLetters 9, 1448-1450 (1997), describes a semiconductor laser structurein which a QW layer (quantum well) made ofGa_(0.6)In_(0.4)N_(0.01)As_(0.99) provided as active layer is arrangedbetween layers made of Ga_(0.97)In_(0.03)N_(0.01)As_(0.99).EP-A-0,896,406 discloses a semiconductor laser structure having anactive layer made of InN_(x)As_(y)P_(1−x−y) (0<x<1 and 0≦y<1) betweenlayers made of GaN_(x′)As_(y′)P_(1−x′−y′) (0<x′<1 and 0≦y′<1). Asemiconductor laser structure having an active layer made ofIn_(y)Ga_(1−y)As_(1−w−v)Sb_(w)N_(v) (v≦0.0095 and w+y≧0.33) betweenlayers made of GaAs_(1−z)P_(z) (0≦z≦1) or In_(y)Ga_(1−y)As (0.53≦y≦1) isdisclosed in U.S. Pat. No. 5,719,894 and U.S. Pat. No. 5,825,796.

[0003] It is an object of the present invention to specify asemiconductor layer structure which is suitable for fabricating laserdiodes and enables efficient radiation emission at wavelengths of 1.3 μmand above. This object is achieved by means of the semiconductor laserstructure having the features of claims 1, 4 and 7, respectively.Refinements emerge from the respective dependent claims.

[0004] The semiconductor laser structure according to the invention isbased on the insight that the radiation emission in a wavelength rangeof 1.3 μm and above can be significantly improved if the properties ofthe barrier layers which bound the active layer provided for radiationgeneration are set more precisely with regard to the strains anddislocations that occur in the heterostructure. To that end, the layerprovided for radiation generation and the barrier layers comprisematerial compositions which contain a III component, a V component and N(III and V corresponding to the groups of the periodic table). Theemission wavelength is set by the nitrogen proportion in the activelayer. In one embodiment, the active layer is quaternary material with aproportion of a further III component, and the barrier layers areternary material; in a further embodiment, the layers are composed ofthe same chemical elements and differ only in the percentage proportionsof these elements (e.g. in each case quaternary material made of thesame elements with different atomic proportions), but the nitrogenproportion is higher in the barrier layers than in the active layer. Inthe case of a preferred exemplary embodiment in the material system ofGaAs, both the active layer and the barrier layers comprise Ga, As andN. The active layer is then preferably InGaAsN, the barrier layers areInGaAsN with a higher nitrogen proportion or GaAsN.

[0005] Another embodiment comprises superlattices in the barrier layers,which are formed by a sequence of thin layers which each contain a IIIcomponent, a V component and N in different percentage proportions. Inthe material system of GaAs, the layers which form the superlattice aree.g. In_(x)Ga_(1−x)As_(y)N_(1−y) with different proportions x and y,where it may be the case that, in particular, x=0 and y=1. Thecompositions of the individual layers are chosen, however, in such a wayas to produce the desired proportion of nitrogen or indium overall inthe superlattice.

[0006] According to the invention, it is possible to achieve, on the onehand, sufficient lattice matching of the grown layers and, on the otherhand, a sufficiently large jump in the energy band gap, as a result ofwhich confinement is effected. The barrier material need not necessarilyoccupy the entire layer thickness of the component above and below theactive layer (in the case of a VCSEL, by way of example, the regionbetween the DBR gratings functioning as resonator end mirrors). In thepractical embodiment, barrier layers having a thickness of typically 50nm suffice; outside there may be GaAs, for example as cladding layer. Amultistage reduction of the energy band gap in the barrier layers mayalso be advantageous.

[0007] An example of the heterostructure according to the invention isdescribed below with reference to the figures.

[0008]FIG. 1 shows a layer construction in cross section.

[0009]FIG. 2 shows an energy diagram for the layer construction of FIG.1.

[0010] In a preferred exemplary embodiment of the invention, the activelayer 1 (see FIG. 1) is, made of InGaAsN, and the adjoining barrierlayers 2 are made of semiconductor material of the same components, butwith a lower indium content and higher nitrogen content. The proportionof indium in the barrier layers can also be completely reduced, so thatthe barrier layers 2 are GaAsN. The structure of a surface emittinglaser diode with a vertical resonator (VCSEL) is illustrated as anexample in FIG. 1. In this case, the required laser resonance isgenerated by upper and lower DBR gratings 3 (distributed Braggreflection). The arrangement is preferably situated on a substrate 4.Further details of the laser diode which, like the connection contacts,are known per se have been omitted in order to illustrate the partsessential to the invention.

[0011]FIG. 2 shows a diagram in which the left-hand side depicts theprofile of the upper edge of the valence band and the right-hand sidethe profile of the lower edge of the conduction band for the layerconstruction illustrated in FIG. 1. The regions applicable to the activelayer 1, the barrier layers 2 and the adjoining gratings 3 aredesignated by the corresponding numerals. The diagram is not drawn toscale, but shows qualitatively correctly the typical relations of theenergy band gaps in the individual layers. It is assumed in this casethat the active layer 1 is InGaAsN, and that the barrier layers 2 arelikewise InGaAsN but with a reduced indium content compared with thematerial of the active layer 1. FIG. 2 depicts using broken lines thecorresponding curve profiles for the case where the barrier layers 2 areGaAs. It can be seen that when InGaAsN is used for the barrier layers 2,a reduced energy band gap compared with the use of GaAs results in thebarrier layers. This reduced energy band gap results, as can be seen inFIG. 2, from the fact that, in the barrier layers 2, the upper edge ofthe valance band is lowered to a smaller extent than the lower edge ofthe conduction band.

[0012] In the case of an arrangement of the semiconductor laserstructure on GaAs, owing to the smaller lattice constant of InGaAsNcompared with GaAs, the active layer is usually subjected to severecompressive strain; this strain could be eliminated in the layer itselfonly by increasing the nitrogen proportion in this layer toapproximately ⅓ of the indium proportion, but that is ruled out owing tothe poor results of the optical quality of the component. By virtue ofthe fact that, in the barrier layers, according to the invention, GaAsNis used or a higher nitrogen proportion than in the active layer ischosen, the barrier layers are strained oppositely to the active layer.

[0013] In the material system described, the DBR gratings provided asreflectors can be fabricated in accordance with conventional layerstructures in the material system of AlGaAs/AlAs. It is equally possibleto provide cladding layers, covering layers or the like made of AlGaAs.What is essential to the layer structure according to the invention isthat both the active layer 1 and the barrier layers 2 adjoining itcontain nitrogen as material component.

[0014] A further configuration of the semiconductor laser structure hassuperlattices in the barrier layers. The mean lattice constant of thesuperlattice is preferably less than or equal to that of the substratematerial in order to avoid additional straining of the layer structure.The mean energy band gap of the superlattice preferably lies betweenthat of the active layer, which forms the potential well with thebarriers, and a cladding layer adjoining the barrier layer in each caseon the side remote from said layer. In this case, it must be ensuredthat an energetic barrier with respect to the active layer is presentfor all charge carriers, electrons and holes. Suitable superlattices canbe formed, to be precise in particular on GaAs as substrate material,e.g. by sequences of layers made of In_(x)Ga_(1−x)As_(y)N_(1−y)or madeof In_(x)Ga_(1−x)As_(y) P_(1−y) with different percentage atomicproportions x and y or by sequences of layers made of InGaAsN andAlGaAsN, GaAsN or GaAs. Further possibilities are sequences made ofInGaAs and GaAsN, GaAsP or InGaP.

[0015] The following may be mentioned, in particular, as advantages ofthe layer structure according to the invention. The strain of thematerial of the barrier layers 2 can be set in such a way that it atleast partly compensates for the generally severely compressive strainof the potential well formed by the active layer between the barrierlayers. This enables higher strains of the potential well (and hencelarger layer thicknesses or higher indium contents) without structuralrelaxation occurring. This enables longer-wave radiation emission thanwith conventional GaAs barriers. By virtue of the smaller energy bandgap of the barrier material according to the invention (compared withGaAs barriers), given an otherwise identically structured potentialwell, the optical transition in the potential well is shifted into thelonger-wave, whereby longer-wave radiation emission is likewiseachieved. By virtue of the incorporation of nitrogen into the materialof the barrier layers, the ratio of the jumps of the energy band gaps atthe boundary with respect to the potential well (layer boundary betweenactive layer 1 and barrier layers 2) can be influenced by means of asuitable choice of the percentage proportion of the nitrogen. While theenergy band gap in the active layer 1 remains the same, the barriereffect (confinement) is increased by lowering the upper edge of thevalance band in the material of the barrier layers. This increases theenergetic inclusion of holes and hence the total rate of electron-holerecombinations in the potential well, which increases the efficiency ofa laser provided with this heterostructure. Through the choice of thecompositions with a higher nitrogen content of the barrier layers or theuse of ternary material in the barrier layers and quaternary material inthe active layer, a strain of the active layer can be at least partlycompensated for in such a way that an efficient radiation yield isachieved even at long wavelengths in the range from 1.3 μm upward.

1. A semiconductor laser structure having an active layer (1) betweenbarrier layers (2), said active layer being provided for radiationgeneration, in which the active layer (1) and the barrier layers (2) arein each case a semiconductor material containing a III component, a Vcomponent and nitrogen, in which the barrier layers (2) are asemiconductor material which has a larger energy band gap than thesemiconductor material of the active layer (1), and in which, forlattice matching of the active layer (1) to the barrier layers (2), thesemiconductor material of the barrier layers (2) contains a higherproportion of nitrogen than the semiconductor material of the activelayer (1).
 2. The semiconductor laser structure as claimed in claim 1,in which the active layer (1) is In_(x)Ga_(1−x)As_(y)N_(1−y), and inwhich the barrier layers (2) are In_(x′)Ga_(1−x′)As_(y′)N_(1−y′) wherey′<y, InP_(y′)N_(1−y′) where y′<y, InAs_(y′)P_(y″)N_(1−y)′_(−y″) wherey′+y″<y, or GaAs_(y′)N_(1−y′) where y′<y.
 3. The semiconductor laserstructure as claimed in claim 1, in which the active layer (1) isGaAsSbN, and in which the barrier layers (2) are GaAsSbN or GaAsN.
 4. Asemiconductor laser structure having an active layer (1) between barrierlayers (2), said active layer being provided for radiation generation,in which the active layer (1) and the barrier layers (2) are in eachcase a semiconductor material containing a III component, a V componentand nitrogen, in which the barrier layers (2) are a semiconductormaterial which has a larger energy band gap than the semiconductormaterial of the active layer (1), and in which, for lattice matching ofthe active layer (1) to the barrier layers (2), the active layer (1) isquaternary semiconductor material and the barrier layers (2) are ternarysemiconductor material.
 5. The semiconductor laser structure as claimedin claim 4, in which the active layer (1) is InGaAsN and in which thebarrier layers (2) are InPN or GaAsN.
 6. The semiconductor laserstructure as claimed in claim 4, in which the active layer (1) isGaAsSbN, and in which the barrier layers (2) are GaAsN.
 7. Asemiconductor laser structure having an active layer (1) between barrierlayers (2), said active layer being provided for radiation generation,in which the active layer (1) and the barrier layers (2) are in eachcase a semiconductor material containing a III component and a Vcomponent, in which the barrier layers (2) are a semiconductor materialwhich has a larger energy band gap than the semiconductor material ofthe active layer (1), and in which, for lattice matching of the activelayer (1) to the barrier layers (2), the barrier layers (2) aresequences of layers of different composition which form a superlattice.8. The semiconductor laser structure as claimed in claim 7, in which thebarrier layers (2) are sequences of layers made ofIn_(x)Ga_(1−x)As_(y)N_(1−y) with different percentage atomic proportionsx and y.
 9. The semiconductor laser structure as claimed in claim 7, inwhich the barrier layers (2) are sequences of layers made of InGaAsN andAlGaAsN, made of InGaAsN and GaAsN, made of InGaAs and GaAsN or made ofInGaAsN and GaAs.
 10. The semiconductor laser structure as claimed inclaim 7, in which the barrier layers (2) are sequences of layers made ofIn_(x)Ga_(1−x)As_(y)P_(1−y) with different percentage atomic proportionsx and y.
 11. The semiconductor laser structure as claimed in claim 10,in which the barrier layers (2) are sequences of layers made of InGaAsand GaAsP or made of InGaAs and InGaP.